Free radical initiated emulsion polymerization reactions for making elastomers, such as styrene-butadiene rubber, butadiene rubber and acrylonitrile-butadiene rubber, often involve use of shortstopping agents to terminate the polymerization reaction. The polymerization reaction is stopped at a predetermined degree of monomer conversion in order to produce a rubber product with the desired characteristics.
U.S. Pat. No. 3,148,225 teaches the use of N,N-dialkylhydroxylamines, such as N,N-dimethylhydroxylamine, as popcorn polymer inhibitors in the preparation of synthetic rubber. Previously and currently used shortstopping agents, however, are known to suffer from one or more drawbacks.
As discussed in U.S. Pat. No. 3,148,225 and European Patent No. EP1083185, N,N′-diethylhydroxylamine (DEHA) is a widely-used shortstopping agent, however, it is relatively volatile and is readily removed with the unreactive monomers during the steam stripping step. This in turn can result in insufficient DEHA present in the resulting latex emulsion to prevent additional free radical polymerization from occurring and can cause Mooney viscosity drift. Furthermore, subsequent emulsion polymerizations that utilize recovered and recycled monomer streams can contain excessive amounts of DEHA that will deactivate a portion of the initiator package and will ultimately require use of greater amounts of initiator. The use of DEHA can also result in the formation of precursors to regulated nitrosamines. DEHA is sometimes combined with non-volatile free radical scavengers such as sodium dimethyldithiocarbamate (SDD) or sodium tetrasulfide (ST) to reduce the Mooney viscosity drift of the resulting latex. However, SDD can lead to the formation of nitrosamine precursors and ST can generate toxic and corrosive hydrogen sulfide.
N-Isopropylhydroxylamine (IPHA) is also widely used as a shortstopping agent and has the advantage of providing excellent Mooney viscosity control while not contributing to the formation of any regulated nitrosamines, as discussed in U.S. Pat. No. 5,384,372. IPHA is capable of providing control of undesired vapor phase polymer formation (popcorn polymer), however, its efficacy can be variable depending on plant conditions.
European Patent No. EP 1 083 185 describes the use of nitrosamine inhibitors with secondary alkylhydroxylamines that are otherwise capable of producing nitrosamines in emulsion and rubber processes. The benefits and drawbacks of IPHA and DEHA are discussed in this patent document.
IPHA or its salts may, for example without limitation, be selected from the group consisting of N-isopropylhydroxylamine, N-isopropylhydroxylamine acetate, N-isopropylhydroxylamine hydrochloride, N-isopropylhydroxylamine sulfate, and mixtures thereof.
A blend of IPHA and hydroxylamine (HA) is presented in International Patent Application Publication No. WO 2002/0038617 as a nitrosamine-free shortstopping agent having better prevention of popcorn polymer formation than IPHA alone. Hydroxylamine, however, rapidly decomposes, particularly in the presence of metal ions, which creates significant storage and handling issues. On the other hand, U.S. Pat. No. 5,504,168 describes blends of IPHA with polysulphides used as shortstopping agents for emulsion polymerization systems.
International Patent Application Publication No. WO 1998/0051714 teaches the use of N-ethylhydroxylamine (EHA) or its salts as a shortstopping agent for free radical polymerization. This document states that EHA is less volatile than DEHA and IPHA and it can be mixed with other shortstopping agents such as IPHA, DEHA, sodium polysulfide, and sodium dimethyldithiocarbamate. Furthermore, EHA does not produce undesirable nitrosamines.
International Patent Application Publication No. WO 2000/0042079 discloses sterically-hindered alkyl hydroxylamines, such as N-tertiary-butylhydroxylamine and N,N-isopropylmethylhydroxylamine, and their use as free radical scavengers and shortstopping agents.
U.S. Pat. No. 6,723,255 describes shortstopping compositions which contain at least one hydrophilic radical scavenger and at least one hydrophobic radical scavenger. It is shown in this document that shortstopping agents having fewer carbon atoms have greater volatility and water solubility.
In view of the foregoing practices and developments, there remains a need for new and improved shortstopping agents which exhibit at least two or more of the following properties: 1) excellent shortstopping/Mooney viscosity control, 2) good storage stability/EH&S profile, 3) do not form detectable levels of regulated nitrosamines, and 4) can provide consistent popcorn polymer control.